This invention relates to air conditioning and dehumidification equipment and more particularly to air conditioning and dehumidification equipment for storage units.
It is well known that traditional air conditioning designs are not well adapted to handle both the moisture load and temperature load of a building space. In conventional air conditioning systems, the cooling capacity of the air conditioner unit is typically sized primarily to accommodate the sensible (dry-bulb temperature) load and corresponding latent (humidity) load at a peak temperature design condition.
However, the humidity load in an enclosed space does not vary directly with the temperature load. Consequently, during the morning and night time hours, the humidity outdoors is approximately the same as during the higher temperatures found throughout the midday periods. Thus, during the cooler periods in the morning and night time, there is a demand for dehumidification but no cooling requirements. Since most of conventional or standard air conditioning designs do not address a separate latent (humidity) load without a sensible load, it results in uncomfortable conditions within the building.
The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) describe comfort zone conditions for human occupancy with a temperature range of about 73-78° F. and a moisture content between about 55-71 gr./lb. or a relative humidity less than about 60%.
Uncontrolled, high relative humidity conditions often lead to more than just uncomfortable conditions. These conditions also support the formation of mold or the generation of other microbes within the building and in the duct work, which can lead to what is known as Sick Building Syndrome. ASHRAE Draft Standard 62-1892 recommends the use of make-up air (but limits levels of relative humidity) to help overcome these problems, which also helps to improve Indoor Air Quality (IAQ) issues. In order to follow the standard, increased dehumidification capacity independent of cooling demands is necessary.
Because of the inability of typical air conditioning equipment to control the high relative humidity conditions found in most buildings, the 1997 ASHRAE Handbook of Fundamentals climate data included peak dew point conditions, as well as peak sensible load conditions, and peak wet bulb conditions. The inclusion of peak dew point conditions allows the air conditioning equipment to be more accurately sized because many geographic regions have a higher Btu/h load at the peak dew point condition than at the previously listed corresponding peak sensible or peak wet bulb conditions.
One solution to the problems associated with typical air conditioning equipment is to design a conditional air conditioning system using a refrigeration circuit that is sized for the total heat load using the climatic design data from the 1997 ASHRAE Handbook of Fundamentals. The air conditioner capacity would be sized based on the highest of either the peak temperature (sensible) condition or the peak moisture (wet bulb and dew point) condition, whichever condition results in the highest total Btu/h requirement. Although this would allow the equipment to control both sensible and latent loads, it would likely over-cool the space and require reheating the supply air to meet the comfort zone conditions.
Another solution is to use a desiccant cooling system. A desiccant wheel or belt is used to remove moisture (latent heat) from an air supply. In typical applications, about 75% of the desiccant wheel is in the target air path as it rotates. The other 25% of the wheel is in a wedge-shaped regeneration chamber. Regeneration is accomplished by passing hot air (usually over 140° F.) through the wheel, which provides a greater attraction for water than the desiccant. This type of system can provide close and independent control of humidity and temperature. The advantage of the system is that it relies on low cost heat sources for the regeneration, thus providing better humidity control and lower overall energy costs than a conventional air conditioning unit. The problem is that desiccant cooling systems do not reduce the energy load. They simply replace latent load with increased sensible (heat) load, i.e., the moist air becomes drier but hotter air.
Industry reports on the self-storage business indicate that most new self-storage facilities include climate controlled units. However, as discussed above, most air conditioning units control temperature (sensible loads) well, but humidity (latent) loads poorly. While dehumidification has been a challenge for human comfort with traditional air conditioning applications (with a relatively small temperature range of 73-78° F.), it is even more difficult for a warehouse with a much wider temperature range (about 50° to about 80° F.).
Ideally, climate controlled warehouse/self-storage units want to hold the temperature between about 50° and about 80° F. and the relative humidity (RH) at about 60% or less. A relative humidity less than about 50% is desired because it eliminates condensation, prevents bacteria growth, mold, and mildew, and stops destructive corrosion. The low moisture level also stops dust-mite reproduction, and discourages pests, such as spiders, fleas, cockroaches, and silverfish. “Total climate control” is the new, more descriptive term used in the self-storage industry to refer to both temperature and humidity control. The Total climate control range for storage units are shown in FIG. 1 over the ASHRAE's comfort zone for winter (heating) and summer (cooling).
Therefore, there remains a need for a system which allows independent control of temperature and humidity.